1. Technical Field
The present disclosure relates to a wiring board for a fingerprint sensor.
2. Background
FIG. 5 is a schematic cross-sectional view illustrating a conventional wiring board 20 for a fingerprint sensor. The conventional wiring board 20 for the fingerprint sensor includes an insulating board 11, a wiring conductor 12, and a solder resist layer 13. The conventional wiring board for a fingerprint sensor is disclosed in Japanese Unexamined Patent Publication No. 2001-46359, for example.
The insulating board 11 has a structure in which a buildup insulating layer 11b is laminated on each of upper and lower surfaces of a core insulating layer 11a. The core insulating layer 11a is made of thermosetting resin including glass cloth. A thickness of the core insulating layer 11a is 150 μm to 200 μm. The core insulating layer 11a has a plurality of through-holes 14. The buildup insulating layer 11b is made of thermosetting resin not including glass cloth. A thickness of the buildup insulating layer 11b is 25 μm to 40 μm. The buildup insulating layer 11b has a plurality of via-holes 15.
The wiring conductor 12 is adhered to the upper and lower surfaces of the core insulating layer 11a, an inner side of the through-hole 14, a surface of the buildup insulating layer 11b, and an inner side of the via-hole 15. The wiring conductor 12 is made of plated copper. A thickness of the wiring conductor 12 is 10 μm to 20 μm.
The wiring conductor 12 formed on the uppermost layer partially serves as an outer strip-shaped electrode 16 for reading a fingerprint. Each of the outer strip-shaped electrodes 16 has a thin strip-shaped pattern, and they are arranged parallel to each other along a first direction. The wiring conductor 12 positioned next to that on the uppermost layer across the uppermost buildup insulating layer 11b partially serves as a fingerprint reading inner strip-shaped electrode 17. Each of the inner strip-shaped electrodes 17 has a thin strip-shaped pattern, and they are arranged parallel to each other along a second direction perpendicular to the first direction. The outer strip-shaped electrodes 16 and the inner strip-shaped electrodes 17 vertically overlap and perpendicularly intersect with each other with the uppermost insulating layer 11b sandwiched between them.
The wiring conductor 12 formed on the lowermost layer partially serves as an external connection pad 18. The predetermined external connection pad 18, outer strip-shaped electrode 16, and inner strip-shaped electrode 17 are connected to each other through the wiring conductors 12.
The solder resist layer 13 is adhered to cover the uppermost and lowermost buildup insulating layers 11b and the wiring conductors 12 formed on their surfaces. The solder resist layer 13 is made of thermosetting resin. The solder resist layer 13 contains dispersed silica powder as a filler. A thickness of the solder resist layer 13 on the surface of the wiring conductor 12 is 10 μm to 20 μm. The upper solder resist layer 13 completely covers the wiring conductor 12. The lower solder resist layer 13 has an opening to expose the external connection pad 18.
When a finger is put on an upper surface of the wiring board 20 for a fingerprint sensor and a voltage is applied to the outer strip-shaped electrode 16, electrostatic capacitance is formed between the outer strip-shaped electrode 16 and a finger surface which are opposed across the upper solder resist layer 13. This electrostatic capacitance is high at a projected portion of the fingerprint, and low at a recessed portion of the fingerprint. Thus, a difference in the electrostatic capacitance is detected by sequentially scanning the plurality of outer strip-shaped electrodes 16 and the plurality of inner strip-shaped electrodes 17 while applying a voltage to them, and the fingerprint can be read by processing the detected difference with an external processor.
The wiring board 20 for the fingerprint sensor is required to be reduced in thickness, in addition to being improved in sensitivity for reading the fingerprint. However, according to the conventional wiring board 20 for the fingerprint sensor, the solder resist layer 13 is made of thermosetting resin containing the silica filler, and its relative permittivity is as low as 3.2 to 3.6 in GHz bandwidth. Therefore, electrostatic capacitance to be formed between the outer strip-shaped electrode 16 and the finger surface is small, and it is difficult to enhance the sensitivity for reading the fingerprint. In addition, the thickness of the core insulating layer 11a is as large as 150 μm to 250 μm to ensure mechanical strength, so that it is difficult to reduce the thickness.